Method and apparatus for voice frequency noise reduction in splitterless ADSL

Abstract

An ADSL modem provides compensation for a nonlinear telephone impedance by predistorting transmit signals such that the voltage across the nonlinear impedance is correct independent of the impedance. Correcting the voltage across the nonlinear impedance reduces noise in the telephone due to intermodulation products generated in the voice band. The modem comprises a transmitter that includes (a) a signal source providing a source data signal and (b) a predistortion circuit for predistorting the source data signal to provide a modem transmit signal for coupling to a transmission loop having a nonlinear impedance coupled thereto. The modem device further includes a receiver for receiving a modem receive signal from the transmission loop. The source data signal comprises source digital words, each source digital word corresponding to a particular voltage level. The predistortion circuit includes a memory for mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired voltage level. The transmitter further includes a digital-to-analog converter for converting the predistortion digital words into the modem transmit signal. A feedback circuit coupled to the loop measures the distortion of the modem transmit signal to provide a feedback signal to the predistortion circuit that is indicative of the measured distortion. The predistortion circuit adjusts the amount of predistortion in response to the feedback signal.

What is claimed is: 
     
       1. A modem device comprising: a transmitter including (a) a signal source providing a source data signal and (b) a predistortion circuit for predistorting the source data signal to provide a modem transmit signal for coupling to a transmission loop having a nonlinear impedance coupled thereto, the predistortion circuit compensating for distortion of the modem transmit signal due to the nonlinear impedance; and   a receiver for receiving a modem receive signal from the transmission loop.   
     
     
       2. The modem device of claim 1 wherein the source data signal comprises source digital words, each source digital word corresponding to a particular analog voltage level, and wherein the predistortion circuit includes a mapping table for mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired analog voltage level and the transmitter further includes a digital-to-analog converter for converting the predistortion digital words into the modem transmit signal. 
     
     
       3. The modem device of claim 1 wherein the transmitter further includes a feedback circuit coupled to the transmission loop for measuring the distortion of the modem transmit signal to provide a feedback signal to the predistortion circuit indicative of the measured distortion, the predistortion circuit adjusting the predistortion in response to the feedback signal. 
     
     
       4. The modem device of claim 3 wherein the modem device includes a source resistor having a first end coupled to the transmitter and a second end coupled to the loop, and wherein the feedback circuit includes a comparison circuit having a reference voltage input coupled through a voltage divider to the first end of the source resistor and a loop voltage input coupled to the second end of the source resistor, the comparison circuit comparing the reference voltage input and the loop voltage input to provide the feedback signal. 
     
     
       5. The modem device of claim 3 wherein the source data signal comprises source digital words, each source digital word corresponding to a particular analog voltage level, and wherein the predistortion circuit includes a mapping table for mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired analog voltage level and the transmitter further includes a digital-to-analog converter for converting the predistortion digital words into the modem transmit signal. 
     
     
       6. The modem device of claim 5 wherein the predistortion circuit includes memory for storing the mapping table, the memory having a first address input for addressing the mapping table with each source digital word to provide a corresponding predistortion digital word on a first data output and a second address input coupled to the signal source through a delay line for addressing the mapping table with a delayed source digital word to provide a corresponding delayed predistortion digital word on a second data output, and a control circuit for adjusting the delayed predistortion digital word in response to the feedback signal, the adjusted digital word being coupled to a data input of the memory to update the memory. 
     
     
       7. The modem device of claim 6 wherein the feedback signal comprises a binary signal having a first level and a second level and wherein the control circuit increments the value of the delayed predistortion digital word in response to the first level and decrements the value of the delayed predistortion digital word in response to the second level. 
     
     
       8. The modem device of claim 3 wherein the source data signal comprises source digital words, each source digital word corresponding to a particular analog voltage level, and wherein the predistortion circuit includes (i) a mapping table for mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired analog voltage level and (ii) a multiplier for multiplying each source digital word with the corresponding predistortion digital word from the mapping table to provide a multiplier digital word and wherein the transmitter includes a digital-to-analog converter for converting the multiplier digital word into the modem transmit signal. 
     
     
       9. A method for communicating a modem transmit signal across a transmission loop having a nonlinear impedance coupled thereto, the method comprising the steps of: providing a source data signal;   predistorting the source data signal to provide a modem transmit signal, the predistortion compensating for distortion of the modem transmit signal due to the nonlinear impedance; and   coupling the modem transmit signal to the transmission loop.   
     
     
       10. The method of claim 9 wherein the step of providing a source data signal comprises providing source digital words, each source digital word corresponding to a particular analog voltage level, and wherein the predistorting step includes mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired analog voltage level; the method further including the step of converting the predistortion digital words into the modem transmit signal. 
     
     
       11. The method of claim 10 further including the steps of measuring the distortion of the modem transmit signal to provide a feedback signal indicative of the measured distortion, and adjusting the predistortion in response to the feedback signal. 
     
     
       12. The method of claim 9 wherein the step of providing a source data signal comprises providing source digital words, each source digital word corresponding to a particular analog voltage level, and wherein the predistortion step includes (i) mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired analog voltage level and (ii) multiplying each source digital word with the corresponding predistortion digital word to provide a multiplier digital word and further including the step of converting the multiplier digital word into the modem transmit signal. 
     
     
       13. A modem device having a transmitter and a receiver, the transmitter comprising: a digital signal processor providing a source data signal;   a predistortion circuit for predistorting the source data signal to provide a modem transmit signal for coupling to a transmission loop having a nonlinear impedance coupled thereto, the predistortion circuit compensating for distortion of the modem transmit signal due to the nonlinear impedance; and   a feedback circuit coupled to the transmission loop for measuring the distortion of the modem transmit signal to provide a feedback signal to the predistortion circuit indicative of the measured distortion, the predistortion circuit adjusting the predistortion in response to the feedback signal.   
     
     
       14. The modem device of claim 13 wherein the source data signal comprises source digital words, each source digital word corresponding to a particular analog voltage level, and wherein the predistortion circuit includes (i) a mapping table for mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired analog voltage level and (ii) a multiplier for multiplying each source digital word with the corresponding predistortion digital word from the mapping table to provide a multiplier digital word and wherein the transmitter includes a digital-to-analog converter for converting the multiplier digital word into the modem transmit signal. 
     
     
       15. The modem device of claim 14 wherein the feedback signal comprises a binary signal having a first level and a second level and wherein the predistortion circuit increments the value of the predistortion digital word in response to the first level and decrements the value of the predistortion digital word in response to the second level. 
     
     
       16. The modem device of claim 13 wherein the source data signal comprises source digital words, each source digital word corresponding to a particular analog voltage level, and wherein the predistortion circuit includes a mapping table for mapping the source digital words to predistortion digital words, each predistortion digital word corresponding to a particular desired analog voltage level and the transmitter further includes a digital-to-analog converter for converting the predistortion digital words into the modem transmit signal. 
     
     
       17. The modem device of claim 16 wherein the feedback signal comprises a binary signal having a first level and a second level and wherein the predistortion circuit increments the value of the predistortion digital word in response to the first level and decrements the value of the predistortion digital word in response to the second level. 
     
     
       18. The modem device of claim 16 wherein the predistortion circuit includes memory for storing the mapping table, the memory having a first address input for addressing the mapping table with each source digital word to provide a corresponding predistortion digital word on a first data output and a second address input coupled to the signal source through a delay line for addressing the mapping table with a delayed source digital word to provide a corresponding delayed predistortion digital word on a second data output, and a control circuit for adjusting the delayed predistortion digital word in response to the feedback signal, the adjusted digital word being coupled to a data input of the memory to update the memory. 
     
     
       19. The modem device of claim 13 wherein the modem device includes a source resistor having a first end coupled to the transmitter and a second end coupled to the loop, and wherein the feedback circuit includes a comparison circuit having a reference voltage input coupled through a voltage divider to the first end of the source resistor and a loop voltage input coupled to the second end of the source resistor, the comparison circuit comparing the reference voltage input and the loop voltage input to provide the feedback signal.